Rename LoadIframeUrlMsg to CreateIFrameAndLoadUrlMsg

This better reflects what the message does.
Small fix to variable name

This is a more accurate name for the script pipeline.
Add some factory methods for frame tree types
Send incremental frame tree updates to the compositor

This allows the compositor to add frames after the call to SetIds,
where the initial frame tree is created. There are still some issues
preventing proper late frame creation, but this prevents crashes when
it happens.

Fixes #3738.
auto merge of #3951 : mrobinson/servo/iframes, r=jdm

This is the first step to allowing incremental iframe creation and destruction. This should eliminate task failures when an iframe is added to the frame tree lazily via script.